Compliant mechanisms are devices that gain their mobility from elastic deformation rather than the rigid-body motions of conventional mechanisms. Unlike traditional rigid-link mechanisms where elastic deformation is detrimental to performance, a compliant mechanism is designed to take advantage of the flexibility of the material. The function of the compliant member within a compliant mechanism can be as basic as serving as a simple spring or as complex as generating a specified motion.
At the micro scale, compliant mechanisms are important because frictional forces encountered in conventional rigid joints dominate the inertial forces at the micro level, thus making the use of rigid-link mechanisms inappropriate for micro applications. Because friction in the micro scale discourages the use of gears and joints due to excessive energy loss, the obvious alternative choice is compliant mechanisms since they do not suffer frictional losses. Compliance is of particular importance to the further development of MEMS because compliant mechanisms reduce part counts when compared with rigid-body mechanisms that produce the same function, thus enabling further miniaturization.
Compliant mechanisms are well suited for MEMS applications because their joint-less, single-piece construction is unaffected by many of the difficulties associated with MEMS, such as wear, friction, inaccuracies due to backlash, noise, and clearance problems associated with the pin joints. In addition, many compliant mechanisms are planar in nature, do not require assembly, and can be made using a single layer. This greatly enhances the manufacturability of micro-mechanisms because MEMS are planar and are typically built in batch production with minimal or no assembly.
A compliant system is considered to be stable, and at a potential energy minimum, if a small external disturbance only causes it to oscillate about an equilibrium position. An equilibrium position is unstable, a potential energy maximum, if a small disturbance causes the system to move to another position. Typical compliant mechanisms have only one stable state, and require a sustained force in order to hold a second state. A bistable mechanism, on the other hand, is capable of holding one of two stable states at any given time, and consumes energy only during the motion from one stable state to the other. This bistable behavior is achieved by storing energy during part of its motion, and then releasing it as the mechanism moves toward a second stable state. Because flexible segments store energy as they deflect, compliant mechanisms can be designed to use the same segments to gain both motion and a second stable state, which can result in a significant reduction in part count.
In MEMS as well as in other applications, there exists a large need for bistable devices, or devices that can be selectively disposed in either of two different, stable configurations. Bistable devices can be used in a number of different mechanisms, including switches, valves, clasps, and closures. Switches, for example, often have two separate states: on and off. However, most conventional switches are constructed of rigid elements that are connected by hinges, and therefore do not obtain the benefits of compliant technology. Compliant bistable mechanisms have particular utility in a MEMS environment, in which electrical and/or mechanical switching at a microscopic level is desirable, and in which conventional methods used to assemble rigid body structures are ineffective